GCN Circular 10505
R.L.C. Starling, P.A. Evans (U. Leicester) and M. Stamatikos (OSU/NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed the first orbit of Swift XRT data for GRB 100316D (Stamatikos et al. GCN Circ. 10496), comprising 593 s of exposure from 143 s to 737 s after the BAT trigger. The data are solely in Windowed Timing (WT) mode. The light curve over this time decays very slowly and can be modelled with a power-law with a decay index of alpha=0.13+/-0.03. A spectrum formed from these data can be fitted with an absorbed power-law with a photon spectral index of 1.42 (+/-0.04). The best-fitting absorption column is 5.9 (+/-0.3) x 10^21 cm^-2, in excess of the Galactic value of 7.0 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.6 x 10^-11 (8.7 x 10^-11) erg cm^-2 count^-1. If the decay contines with alpha=0.13, the predicted count rate at T+24h is 16 count s^-1 or an observed (unabsorbed) 0.3-10 keV flux of 1.1 (1.4) x10^-9 erg cm-2 s-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00416135. This circular is an official product of the Swift-XRT team.